The present invention relates to the field of electronically controlled, optical switching elements, especially for use in fiber optics communication systems.
Fast multidimensional switches are essential building blocks in high speed data communication systems, multimedia services, or high performance parallel computers. However, electronic implementations of such switches are close to their inherent limits. It is evident that it will not be possible to meet the demands of the emerging broadband communication applications by the existing electronic switching technology. Furthermore, electronic switching devices are not capable of direct integration with the optical fiber communication systems, which are becoming the dominant communications technology. Optical implementation of switching devices possess several inherent advantages over their electronic counterparts.
Free space photonic interconnects are very suitable for the implementation of parallel communication systems due to the small cross-talk between the parallel data channels, and the case whereby three dimensional guiding and routing can be achieved. The cross-talk level must be kept as low as possible in order to reduce the bit error rate (BER) of the channel. One of the basic building blocks in the construction of such networks is the high speed optical switch, such as the bypass-exchange switch. Such switches are used as the elements in Multistage Interconnection Networks (MIN). MIN""s are a family of network architectures with minimal number of switches.
Many optical MIN configurations have been proposed for highly parallel communication channels with high bandwidth and low cross talk, such as those described by J. W. Goodman, F. I. Leonberg, S. Y. Kung, and R. A. Athale in their article entitled xe2x80x9cOptical interconnection for VLSI systemsxe2x80x9d, published in Proceedings of the IEEE, Vol. 72, pp. 850-866 (1984). Many of these configurations are static optical interconnects which are built either of optical waveguides, such as described by S. Somekh, E. Garmire, A. Yariv, H. L. Garvin and R. G. Hunsperger, in their paper entitled xe2x80x9cChannel optical waveguides and directional coupling in GaAs-imbedded and ridgedxe2x80x9d, in Applied Optics, Vol.13, pp.327-330 (1974), or of imaging systems such as that described by A. W. Lohman, W. Stork and G. Stucke, in xe2x80x9cOptical perfect shufflexe2x80x9d, published in Applied Optics, Vol. 25, pp.1530-1531 (1986), or of static holographic optical elements (HOE""s) such as that described by R. K. Kostuk, J. W. Goodman and L. Hesselink, in their article xe2x80x9cDesign considerations for holographic optical interconnectsxe2x80x9d, published in Applied Optics, Vol. 26, pp.3947-3953 (1987). However, dynamic optical interconnects, which enable the dynamic reconfiguration of the connecting scheme between the source nodes and the target nodes are overwhelmingly more effective, as described in the articles in the review volume entitled xe2x80x9cPhotonic Switching and Interconnectsxe2x80x9d by Abdellatif Marrakehi, published by Marcel Dekker Inc., 1994.
The structure of a MIN is usually of alternating layers of static interconnection patterns, such as the perfect shuffle described by H. S. Stone in xe2x80x9cParallel processing with perfect shufflexe2x80x9d, IEEE Transactions on Computing, Vol. C-20, pp. 152-161 (1971), followed by an array of basic switching modules, known as bypass-exchange switches. The bypass-exchange switch has two inputs and two outputs with two operating states: the bypass state, in which the two input signals are directly connected to the respective output ports and the exchange state, in which the input signals are crossed between the output ports.
A common optical exchange-bypass switch currently in use utilizes a Polarizing Beam Splitter (PBS) combined with a polarization control element at the input to the PBS. The polarization control element is usually a liquid crystal, or a ferroelectric liquid crystal, which is faster. However, even the ferroelectric liquid crystal does not have a sufficiently fast response time for the requirements of present communication switching needs, and certainly not for future needs. Moreover, another major drawback of PBS is the high sensitivity of the cross-talk level of the switch to polarization instability of the transmitted light or the liquid crystal modulators. Consequently, the PBS cross-connect switch is sensitive to the temperature and environmental stability of the modulators and of the whole system.
Holographic optical elements (HOE""s) and volume holograms have been used recently for two dimensional steering of light beams in optical interconnect networks, especially for highly parallel computer interconnects. However, such systems have generally relied, at least in the case of volume holograms, either on the use of a number of fixed holograms, the desired one of which is reconstructed using a reference beam selected by means of its wavelength or direction of incidence, or on the rewriting of the desired hologram in real time immediately before each steering action to be performed. Therefore, such holograms are not directly electrically switchable, and thereby do not provide for simple system construction and high speed operation.
With the increase of the bit throughput rate in optical fiber communication systems, cost effective light sources with very narrow spectral linewidths have been developed. The development of such lasers for optical communications has enabled the use of volume (thick) holograms as routing devices. Since such holograms are inherently extremely wavelength sensitive, their use had not previously been feasible commercially. The use of thick holograms now enables the packing of many routes in the same network, and thus allows three dimensional steering. However, to date, optical switches based on the use of prior art holograms, since they are not directly electrically switchable, have not shown sufficient speed, nor do they possess sufficiently low cross-talk levels, to enable their use in the optical communication systems currently under use or development.
There therefore exists a serious need for fast, dynamic, low-cross talk optical switches, to fulfill the switching requirements in current and future optical communications systems.
The present invention seeks to provide a new free-space optical switch, which overcomes the drawbacks and disadvantages of existing switches.
There is thus provided in accordance with a preferred embodiment of the present invention, a novel generic switch based on Electro holography (EH). EH enables the reconstruction process of volume holograms to be controlled by means of an externally applied electric field. EH is based on the use of the voltage controlled photorefractive effect in the paraelectric phase, where the electro-optic effect is quadratic. Volume holograms stored as a spatial distribution of space charge in a paraelectric crystal can be reconstructed by the application of an electric field to the crystal. This field activates prestored holograms which determine the routing of data-carrying light beams.
The implementation of EH based devices requires the use of a photorefractive crystal with suitable properties, such as potassium tantalate niobate (KTN), strontium boron niobate (SBN), or especially potassium lithium tantalate niobate (KLTN), as described in U.S. Pat. No. 5,614,129. KLTN doped with copper and vanadium is particularly suitable for use as the medium for EH devices.
EH devices can be advantageously used as the building blocks in Multistage Interconnection Networks (MIN). The MIN is composed of arrays of EH switches which can be electrically switched between one or more states. In each state a different set of holograms are activated, which direct the light beams in the required 3D directions to the next stage. These switches thus contain the spatial routing information, thereby obviating the need for additional optics between the stages. The EH switch thus enables a wide variety of interconnect configurations to be implemented, with compact dimensions and for large number of nodes.
Furthermore, unlike conventional holographic memory components based on conventional photorefractive crystals, which can be written and read only in the visible, the EH devices based on KLTN and similar materials can be operated in the near infra-red regions of the spectrum, including at 1.3 xcexcm and 1.55 xcexcm, wavelengths which are now commonly used in standard communication systems.
The use of EH switching technology can extend the routing capabilities of the basic bypass-exchange or cross-connect switch by increasing the number of input and output ports. Consequently, the use of EH switches according to the present invention enables a significant reduction in the total number of switches required for a full access MIN, thereby significantly decreasing the system size and cost.
There is thus further provided in accordance with another preferred embodiment of the present invention, an EH voltage-controlled optical switch, consisting of a paraelectric photorefractive material, wherein is stored a hologram whose reconstruction is controllable by means of an applied electric field.
In accordance with still another preferred embodiment of the present invention, there is provided an optical switch consisting of a paraelectric photorefractive material, wherein is stored a latent hologram whose activation and reconstruction is controllable by means of an applied electric field.
There is further provided in accordance with yet another preferred embodiment of the present invention, an optical switch as described above, and wherein the hologram is formed by spatial modulation of the reference index of the paraelectric photorefractive material.
There is further provided in accordance with still another preferred embodiment of the present invention, an optical switch as described above, and wherein the hologram is formed by spatial modulation of the refractive index of the paraelectric photorefractive material and is in the form of a set of at least one grating.
There is provided in accordance with still a further preferred embodiment of the present invention, an optical switch as described above, and wherein the spatial modulation of the refractive index of the paraelectric photorefractive material arises from the quadratic electro-optic effect, induced by the combined action of a spatially modulated space charge within the paraelectric photorefractive material and an external applied electric field.
Furthermore, in accordance with yet another preferred embodiment of the present invention, there is provided an optical switch as described above, and wherein the spatial modulation of the refractive index of the paraelectric photorefractive material arises from the quadratic electro-optic effect induced by the combined action of a spatial modulation of the low frequency dielectric constant within the paraelectric photorefractive material and an external applied electric field.
There is even further provided in accordance with a preferred embodiment of the present invention, an optical switch as described above and wherein the photorefractive material is a crystal of doped Potassium Lithium Tantalate Niobate.
There is also provided in accordance with a further preferred embodiment of the present invention, an optical switch as above, and wherein the electric field is applied by means of electrodes on two opposite faces of the photorefractive material.
In accordance with yet another preferred embodiment of the present invention, there is provided an optical switch consisting of at least two paraelectric photorefractive crystals, in each of which is stored at least one hologram, whose reconstruction is controllable by means of an electric field applied to each of the crystals, the crystals being disposed so that a light beam traverses them serially.
In accordance with a further preferred embodiment of the present invention, there is also provided an optical switch consisting of at least two paraelectric photorefractive crystals, in each of which is stored at least one latent hologram, whose activation and reconstruction are controllable by means of an electric field applied to each of the crystals, the crystals being disposed so that a light beam traverses them serially.
In accordance with a further preferred embodiment of the present invention, there is also provided an optical switch consisting of at least two paraelectric photorefractive crystals, as described above, and wherein each of the at least two photorefractive crystals diffracts at least one input light beam to a preselected output direction, in accordance with the at least one hologram stored therein.
In accordance with still another preferred embodiment of the present invention, there is provided an optical switch consisting of at least two paraelectric photorefractive crystals, as described above, and wherein the input light beam can be switched to a preselected output direction according to the electric field applied in each of the at least two photorefractive crystal.
There is further provided in accordance with yet another preferred embodiment of the present invention, an optical switch consisting of at least two paraelectric photorefractive crystals, as described above, and wherein undiffracted light is either absorbed by a light block or is inputted to a detector.
There is further provided in accordance with still another preferred embodiment of the present invention, an optical switch including a detector as described above, the switch being part of an optical switching network, and wherein the detector is used to read the address header of optical data traversing the switch, and wherein the address header could be used to control the switching network.
There is provided in accordance with still a further preferred embodiment of the present invention, an optical switch as previously described, and wherein the hologram is written at a visible wavelength, and the hologram is reconstructed at a near infra-red wavelength.
Furthermore, in accordance with yet another preferred embodiment of the present invention, there is provided an optical switch according to any of the above described embodiments, and wherein any arbitrary direction of each of the incoming and outgoing light beams can be defined by writing the appropriate set of holograms into each crystal.
There is even further provided in accordance with a preferred embodiment of the present invention, a switching network, which could be a multistage network, for use in an optical communications system, and incorporating at least one optical switch according to any of the above embodiments.
There is also provided in accordance with a further preferred embodiment of the present invention, a switching network for use in an optical communications system, as described in the previous embodiment, and wherein the switch layer and the static interconnection layer are integral.
The disclosures of all publications and the patent mentioned in this section and in the other sections of the specification, and the disclosures of all documents cited in the above publications and patent, are hereby incorporated by reference.